(a) Field of the Invention
The present invention relates to a semiconductor laser device and, more particularly, to an improvement of a semiconductor laser device having a non-current area at an end portion of an active stripe region.
(b) Description of the Related Art
An optical amplifier having an erbium (Er)-doped optical fiber is widely used in recent years. A high output power semiconductor laser which lases at a 1480 nm or 980 nm wavelength is generally used as a light source for excitation of the optical amplifier of this type. The demand for the high output power laser devices has risen more and more due to the wider range usage of the high output power laser.
It is generally known that the laser facet melts down when the semiconductor laser is driven at a high output power. This melt down phenomenon is called catastrophic optical damage (COD), and suddenly stops the operation of the laser device. The COD phenomenon is one of the factors to obstruct the higher output power of the semiconductor laser device used for excitation of the optical amplifier.
The mechanism for the generation of the COD is considered as follows. When a facet of the active layer in the semiconductor laser device rises in temperature, the bandgap of the active layer narrows in that portion, which in turn absorbs more light due to the narrowed bandgap to further rise in temperature. The temperature rise thus generated accelerates the deterioration of the facet of the active layer. In short, non-radiation recombination center formed in the facet is the core of the generation of the COD.
One of the measures for prevention of a COD failure is, as shown in FIG. 1, to provide a small area 51 in the vicinity of the output facet or both the facets of the semiconductor laser, in which current is not injected. The small area may be 25 .mu.m in length as viewed in the direction of the optical axis of the stripe active layer 52. Thus, since there is no non-radiation recombination current component in the laser device which has the non-current area 51 in the facet, temperature rise is prevented at the facets, thereby maintaining a high resistance against the COD failure.
In general, when the current injected in the active layer is increased, the peak gain of the laser gradually shifts toward the lower wavelength, which causes a so-called "mode hopping" phenomenon, wherein the lasing mode discontinuously jumps to an adjacent Fabry-Perot mode. The wavelength interval by which the lasing mode hops generally coincides with the mode interval defined by the laser length.
However, in the semiconductor laser device which has the above-mentioned non-current area, it is often observed that the interval of the mode hopping reaches dozens of times the mode hopping defined by the laser length. In other word, the laser device which does not have the non-current area jumps toward a lower wavelength by a small amount with the increase in the injected current, whereas the semiconductor laser device which has a non-current area does not show change in the lasing wavelength for a longer period and then shows a sudden, large mode hopping, thereby causing an unstable lasing mode with respect to the specified injected current. The unstable lasing mode is an obstacle against the employment of the laser device as an excitation source of an optical amplifier.